<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{
            padding: 0;
            margin: 0;
            list-style: none;
        }
        ul{
            display: flex;
        }
        li{
            border: 1px solid #000;
            margin: 10px;
        }
        li img{
            width: 50px;
            height: 50px;
        }
        .left img {
            width: 300px;
            height: 300px;
        }
    </style>
</head>
<body>

    <div class="left">
        <img src="../images/big.jpg" alt="">
    </div>
    <ul>
        <li>
            <img src="../images/small1.jpg" alt="" data="../images/big.jpg">
        </li>
        <li>
            <img src="../images/6.jpg" alt="" data="../images/6.jpg">
        </li>
        <li>
            <img src="../images/7.jpg" alt=""  data="../images/7.jpg">
        </li>
    </ul>


    <script>


        var oImgs = document.querySelectorAll('li img') ;
        var oLeftImg = document.querySelector('.left img') ;

        var oUl = document.querySelector('ul') ;

        oUl.addEventListener('mouseover' , function(e) {
            if(e.target.tagName === 'IMG') {
                // console.log(e.target);
                // oLeftImg.src = e.target.src ;
                var src = e.target.getAttribute('data') ;
                oLeftImg.src = src ;
            }
        })

    </script>
    
</body>
</html>